Normal sinus rhythm of the heart begins with the sinoatrial node (or "SA node") generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular (AV) groove. This propagation causes the atria to contract.
At the atrioventricular (AV) groove, the impulse encounters the so-called "skeleton" of the heart. Here, a fibrous structure separates the atria from the ventricles. The rings or annuli of the tricuspid valve (between the right atrium and right ventricle) and the mitral (or bicuspid) valve (between the left atrium and the left ventricle) are attached to this fibrous skeleton.
The fibrous skeleton is electrically inert. It normally acts as an insulator to block the conduction of the impulse from the AV node. The electrical impulse would be prevented from crossing over to the ventricular side of the AV groove, if not for the specialized AV conducting tissue, called the atrioventricular node (or "AV node") and the bundle of HIS (or "HIS bundle").
The AV node slows the conduction of the impulse to the ventricles, allowing the atria to first complete their contraction and empty blood from the atria into the ventricles. The slowed impulse eventually enters the HIS bundle, which delivers the impulse to the ventricular side. The ventricles then contract.
The AV conduction system results in the described, organized sequence of myocardial contraction.
Normally, the AV conduction system is the only way for electrical impulses to be conducted from the atria to the ventricles. However, some people are born with additional electrical conduction paths between the atria and ventricles. These extra connections are called "bypass tracts" or "accessory pathways." Accessory pathways consist of tiny bands of myocardial tissue that most commonly insert in atrial muscle on one end and ventricular muscle on the other end. The most common variety is located along the AV groove.
Accessory pathways offer a potential parallel route for electrical impulses, bypassing the normal AV conduction system.
The accessory pathways do not slow down the electrical impulse, like the AV node does. Instead, the accessory pathways conduct impulses more quickly, like myocardial tissue. When they conduct the impulses in the antegrade direction (i.e., from the atria to the ventricles), they precede the normal impulse from AV node, causing premature stimulation and contraction of the ventricles. When they conduct the impulses in the retrograde direction (i.e., from the ventricles to the atria), the atria contract after the ventricles do. In either case, normal heart rhythm becomes disrupted.
Patients with accessory pathways are susceptible to reentrant tachycardias involving both the AV node and the accessory pathway. the resultant fast heart rate can be potentially life-threatening. The elevated heart rate can lead to serious hemodynamic compromise. Sudden syncope or hemodynamic collapse can occur.
Accessory pathways are generally invisible to the naked eye. They therefore must be located by their electrophysiologic effects. Catheter-based techniques have been developed to record accessory pathway activation by mapping along the AV groove. The conventional mapping techniques typically use a pair of bipolar sensing electrodes to record activation potentials. The sensing electrodes are carried by catheters introduced by vascular access into the heart. These catheter-based techniques have allowed identification of the site of the accessory pathway. Once identified, the conduction block caused by the accessory pathway can be cleared by catheter-based thermal ablation techniques.
FIG. 1 shows a typical electrogram showing the initiation of a normal atrial complex A followed by the initiation of a normal ventricular complex V. FIG. 1 also shows a typical complex associated with an accessory pathway AP, which occurs between the atrial complex A and the ventricular complex V. FIG. 1 shows how relatively small the AP complex is, compared to the atrial and ventricular complexes A and V. FIG. 1 also shows how relatively closely spaced in terms of time (measured in milliseconds) the AP complex is to the ventricular complex V. This time difference is typically only about 20 to 36 milliseconds.
For these reasons, physicians frequently find it difficult to locate the accessory pathway (AP) activation potentials using conventional bipolar sensing techniques.
Conventional bipolar electrodes are not very sensitive to small volume, far-field signals, like those associated with accessory pathways. Often, the AP activation potentials become fused with the local ventricular potentials V. As a result, physicians cannot differentiate between the potentials with enough certainty to positively locate the site of the accessory pathway.
Furthermore, far-field signals will be missed if the bipolar electrodes are oriented perpendicular to the signal path. As a result, many complex movements are presently required to map the AV groove, as the physician must continuously change the orientation of conventional bipolar electrodes to assure that the far-field activation signals are not missed. The difficulties involved in manipulating the electrodes within the heart are significant, as they must be controlled remotely while relying on using indirect fluoroscopic imaging. Stable and intimate contact between the myocardial tissue and the mapping electrodes are often difficult to achieve and maintain.
As a result, the location of accessory pathways using conventional catheter-based techniques are difficult and time consuming. For these reasons, many attempts at creating curative lesions ultimately fail.
There is a need for catheter-based systems and methods that permit the physician to record multiple electrical events at different relative orientations within a localized area without continuous positioning and repositioning.
There is also a need for catheter-based systems and methods that permit the physician to detect a small volume, far-field signal (like one associated with an accessory pathway), even in the presence of large volume, near field signals (like those associated with atrial and ventricular potentials).